1. Field of the Invention
This invention relates to methods and apparatus for determining the DC content in an AC waveform and has particular application to determining the DC content in the AC waveforms generated by single or multiphase inverters.
2. Description of the Prior Art
It is often important or desirable to determine the presence and the magnitude of any DC content in an AC waveform. Such a DC component is often generated in inverters where differences in the characteristics of the paired electronic switches generating the AC waveform result in the generation of a DC component in the inverter output. This DC component, if excessive, can cause extremely high DC circulating currents in AC loads such as motors and transformers. Hence, the DC content must be controlled and also sensed by protection circuits in the event that the controls fail.
It is known to sense the DC content in an inverter output by applying the AC waveform to series connected operational amplifier integrators which attenuate the AC component and amplify the DC component. Since the DC component can be positive or negative, the output of the integrators is applied to a full wave rectifier bridge and the rectified signal is compared with both positive and negative reference voltages. In a multiphase system, the line to neutral voltage of each phase is passed through a pair of integrators with the intergrator outputs all being applied to the full wave rectifier bridge. In this manner, a positive or negative output signal is generated if a DC content of either polarity in any of the phases exceeds the preset limits.
It is an important object of the present invention to provide methods and apparatus for DC content sensing which are less complex, and less costly than those offered by the prior art. Other objects of the invention will become evident from the discussion which follows.